Presentation on, “TEA IMPROVEMENT” By, Shivanand M R Jr. MSc.
Introduction Tea is the most favorite beverage of the people world over. The history of tea dates back to 2737 BC. It is believed that the tea originated in China. According to a Chinese legend, Shen Nung, the second emperor of China, who is considered as the father of agriculture and the inventor of Chinese herbal medicine, invented tea accidentally. The story goes like this. One day while he was sitting in the garden some leaves fell in to his cup of water, the emperor happened to taste that water which had a nice aroma and flavor, thus the tea leaves was discovered.
Tea is an important plantation crop and native of South East Asia. India is the largest producer, consumer and exporter in Tea industry, India has 4,36,057 ha area under tea, of which 87,993 ha is in the south. As on today, 38 countries grow Tea and among which India, China, Sri Lanka and Indonesia have major share in area and production. Major states producing Tea in India are, Assam(53%), West Bengal(24%), Tamil Nadu(13%),Kerala(8%),Karnataka(0.7). Tea was first planted on a large scale in North India in 1834, while in South India commercial tea planting was started at various tea growing regions like Nilgiris (1859), Central Travancore (1875), Kannan Devan Hills (1878), Wynad (1859) and Anamalais (1897).
Tea research was started in North India during 1960 and in South India during 1926. "Tea" refers to the aromatic beverage prepared from the cured leaves by combination with hot or boiling water, and is the common name for the Camellia sinensis plant itself. After water, tea is the most widely consumed beverage in the world. It has a cooling, slightly bitter, astringent flavour which many enjoy.
Botany Tea belongs to the genus Camellia and family Camelliaceae or Theaceae, Tea is a diploid i.e (2n=30). This genus accommodates as many as 82 species (Sealy, 1958) and over 200 species (Zhijian et a l., 1988). But only two species viz., C. assamica (Assam jats) and C. sinensis (China jats) are the original species for tea. As they were highly crossable with each other, the present day tea seedlings are hybrids of these two species and are often referred as C. assamica sub sp. Lasiocalyx (Indo-China or Cambod type) The commercially cultivated tea consists natural hybrids of all these three types and there is reason to believe that complex hybrids of these three types of tea and one or more of the wild species of Camellia occur in the cultivated populations.
Tea is essentially an out breeding crop and its races are highly infertile. As a -result of enormous natural hybridization not only between the tea species but also with Camellias and large,-scale dispersal of tea seeds in south-east Asia over the centuries, resulted in extreme heterogeneity in the existing seedling population. Tea taxonomy is very complicated, and hybridization early in its commercial history has left an almost total absence of pure plants (Willson, 1999).
Botanical Composition of Jats The term jat in tea nomenclature is considered close to variety. The multi-stemmed China type has two jats - Low-jat and High jat. The former jat has narrow leaves, profusely indented margin and flat, rather dull-coloured laminae, whereas later has comparatively larger leaves, wider in proportion with blunter tips, fewer serrations and a crinkled surface (Subramanium, 1995). The single-stemmed Assam type has Burma, Lushai, Manipuri, light-leaved Assam and the dark-leaved Assam jats. Burma jat comes from Myanmar hills and has spiny, crinkled and slightly erect leaves ,and has dark- green, older leaves having a bluish tinge.
Lushai is form of Lushai hills and has pendent flabby leaves. Manipuri is a hardy, and high yielder of quality tea. The leaves are dark-green, shiny, rather narrow and have a semi-upright leaf pose. It is softer than Burma jat. The light-leaved Assam is found in the Naga hills and has a semi-upright, glossy, crinkled, light green leaves. The Lushai and the light-leaved Assam are generally not planted in Assam because the former is very delicate and the later is poor yielder and has a balsam-like odour. Dark-leaved Assam has soft, glossy, crinkled and slightly pendent leaves (Subramanium, 1995).
Biology of the Plant Vegetative Structure Under natural conditions, the tea plant is an evergreen tree and wildly grows into a medium size tree. Under cultivation it is pruned down and trained as low spreading bush to ensure that a maximum crop of young shoots can be obtained and maintained. Leaves are simple, alternate and serrated. The root system of a tea bush can be divided into main roots, subsidiary roots and feeder roots. The lateral roots give rise to a surface mat of feeding roots which lack root hairs when mature. Root system varies in tea bushes depending on genetic makeup and soil environment. Starch is stored in roots.
Reproductive Structure Tea flowers appear in the axils of scale leaves, either solitary or in clusters, with short pedicels, fragrant; calyx persistent with 5-7 sepals; petals 5- 7, white, waxy, obovate, concave; stamens numerous with 2-celled yellow anthers; petal and outer stamens united for a short distance at base ovary superior, hairy, with 4-6 ovules per carpel; styles short, free to almost completely fused, stigmatic lobs 3-5. Fruits are capsule thick walled, brownish green, 3-lobed and usually 3-celled, becoming slightly rough at maturity, dehiscing during the dry period by splitting from apex into 3 valves.
Plant Improvement Introduction The tea plant is predominantly cross pollinator and highly heterogenous. It was observed that tea sets better with pollen from another bush, the average set of the plant with its own pollen being about one quarter of that obtained by cross pollination (Wight and Barua, 1939). Further, selfing results in smaller seeds with reduced germinability or no seeds at all (Mamedov, 1961; Sebastiampillai, 1963). Breeding in the strict sense and evolution of pure line races in the polymorphic species Camellia sinensis are time consuming and be set with practical difficulties. Fortunately, a large variation in several important and desirable characters occurs from bush to bush in the existing tea populations.
Exploitation of such plant variability by careful selection for high yield, excellence of cup characters of made tea, fair resistance to drought and some important pests and diseases and adaptations for different environment appear to offer a ready means of developing improved planting material in this perennial crop. Other objectives of breeding vary according to the requirements of different regions. The achievements of all desirable objectives simultaneously will not be practicable but some of these can be realized at a time through the selection of novel plants and their multiplication as vegetative clones or by breeding improved strains of seeds.
Selection Procedure Selection of mother bush from the existing population is most important for production of improved seed and clonal varieties or hybrids. The fact that a tea bush in a field appears vigorous and apparently healthy does not mean much because it may produce huge crop of inferior tea, or a less amount of crop of superior tea. The bush population even in a small field of tea shows great variation in regard to growth habit, inherent quality and yield. Wight (1939) observed that about 10 percent of the bushes in a commercial plantation in north eastern region produce only 2 percent of the total crop and about 0.5 percent bushes produce as much as or more than 300 percent of the yield of an average bush.
Development of Clonal Varieties In seed-grown tea, existence of wide variation are well known to the tea breeders and this probably made them aware of the advantages of clonal propagation for establishing large and uniform population. Clones are plants genetically identical to the parent originating by vegetative propagation such as by leaf cuttings or grafts taken from tea bush. Possibilities of propagation by single leaf and internode cuttings was explored in India (Tunstall, 1931), Sri Lanka (Tubbs, 1932) and Indonesia (Wellensick, 1938a, 1938b). The technique streamlined subsequently, assisted in the rapid multiplication of selected mother bushes leading to the development of clones (Kehl, 1963; Venkataramani, 1970; Wight, 1960). This encouraged tea breeders to develop clonal varieties. Clonal selection also involves considerable time and is a laborious process but widely adopted method of plant improvement in tea.
It usually takes 7-10 years before a clone could be released for commercial planting (Barua, 1963b; Bezbaruah, 1968). The initial selections were done on the basis of colour and size of shoot, pubescence, branching habit, bush area, recovery rate following pruning etc. In clones, rooting character to a great extent is inherent and three types of root systems viz. shallow or horizontal, vertical and intermediate are commonly known. In the shallow and horizontal root system, all roots develop at the surface and run almost parallel to the ground which is obviously undesirable in a perennial rain- fed crop like tea. In the vertical system, one of the roots tend to go down almost vertically, gradually thickening and functioning as tap root.
This type is rarely found. The intermediate root system is most commonly come across and in this type at least one of the root grows at an angle of 30 to 45 from the horizontal and penetrates deep into the soil (Venkataramani, 1970). The development of superior clones with high yield and better quality has been one of the major achievements of the Tocklai breeding programme. UPASI Tea Scientific Department and R & D Department, Tata Tea Ltd as recombinant genetic variability has been fixed.
UPASI scientific Department stated that the initial clonal selection may be eye judgment by examining the bushes for such features as recovery from pruning, wood formation, branching habit, flushing behaviours, colour and size of leaves. In brief, the eye judgment could be based on the following features.1) The number of stems per unit area of the bush should be high and their distribution should be uniform over the entire area of the bush.2) The individual stem should not be thin and whippy and it is also important that variation in thickness of the stems should not be high.
3) The size of the bush need not necessarily be large but consistent with factors detailed above: a bush, showing a tendency to fill in quick with an early lateral spreading habit is desirable. These characters can be determined just before or at the time of pruning or in the first year from pruned.4) The density of plucking points per unit area should be high.5) Uneven growth of the shoots is not desirable and it is therefore necessary to select for evenness of bud break and growth of shoots.6) The main branch and the primaries developing from them after pruning should have straight or erect growth and not oblique or horizontally spreading habit.7) The bigger the leaf, the larger the weight of the shoot and hence greater the yield.
8) There seems to be a relationship between the colour of the leaf and cup quality, light coloured leaves generally produce better tea than dark coloured ones.9) Selection may also be made for pubescence of buds and leaves, a selection with pubescence flush generally produces tea with better quality than with glabrous flush.10) Resistance to blister and other diseases and pests and drought are some of the factors to be considered in selection for yield, it would be useful to compare the yield per unit area of plucking surface of the selection bush with the yield per unit area of the field during the period of experimentation (at least three continuous months). The ratio is termed yield potential. The greater the ratio the greater is the potentiality for yield. Yield potential= Bush yield per unit area Field yield per unit area
Among these, clones such as UPASI-3, UPASI-8, UPASI- 17, UPASI-25, UPASI-27, UPASI-28 and TRF-1 are high yielders. Clones such as UPASI-2, UPASI-6, UPASI-9, UPASI-20 and UPASI-26 and the TRF 3 are moderate yielders but are tolerant to drought. Certain Sri Lankan clones such as TRI-2024, TRI-2025 and TRI 2026 and a few estate clones such as CR- 6017, C-1, ATK-1 and SA-6 are also widely used for planting in southern India. Certain clones such as CR 6017, UPASI 3, TRF 2, are known for their quality. Current clonal selection work in the Anamallais, Nelliampathy and Nilgiris resulted in the selection of 140 bushes based on visual examination. Studies are in progress to monitor their nursery and field performance .
BICLONAL SEED STOCKS Besides the clones, five biclonal seed stocks have also been released for commercial planting. The biclonal seed stocks are UPASI: BSS-1 to BSS-5. These seed stocks germinate early in the nursery and establish well in the field. They are good for infilling, replanting and extension planting. The stocks are moderate yielders but are tolerant to drought. Seed bari blocks for all the five seed stocks have been established in the Research Institute and in other planting districts for supplying seeds to the tea growers. In the current hybridization programme crosses were made between UPASI -9 and the drought tolerant clone C- 1, blister resistant clone SMP-1 and the quality clone CR- 6017. Progenies derived out of all these crosses were planted in the field for studying their field performance.
Hybridization In tea, due to self-sterility, pure lines breeding by conventional techniques is not possible . Tea can be genetically upgraded through hybridization. This can be done by transferring desirable characters from the parents by hybridizing them to develop seed cultivars. The seed obtained from crosses between two clones is known as biclonal seed which show considerable variation in genotypic characters while retaining a certain amount of phenotypic uniformity. Such inherent variation is important for the development of clones, which in turn could be used in evolving seed stocks.
After evaluation of germinability and growth performance of biclonal progenies in the nursery on the basis of height, stem diameter, root and shoot ratio; the field performance of the progenies are evaluated in the same manner as that described under clonal selection. Hybridization between genetically diverse parents introduces improved hybrid vigour which often reflected higher yield in the progenies. Therefore, a clone-seed cycle is important for continued plant improvement and its appropriate commercial exploitation (Banerjee, 1992; Satyanarayana and Sharma, 1993).
Hybridization also helps in broadening the genetic base as genes from plants with different genetic linkages could be brought together. However, only a limited success has been achieved in developing cultivars through sexual gene transfer because of certain limitations such as large size of the plant and long-duration, generative cycles ranging from 5 to 10 years (Subramaniam, 1995). Production of synthetic seed varieties was one of the early breeding efforts at Tocklai, Assam. A group of selected clones were planted in isolated seed orchards to produce the seed under natural conditions. Such seed are commonly known as polyclonal seed. These seeds were found to be of unpredictable performance and more unstable than biclonal seeds (Singh, 1982).
Interspecific Hybridization In tea natural interspecific hybridization between different Camellia species are not common and limited attempts have been made for artificial induction of interspecific hybrids. Wight and Barua (1957) attempted interspecific hybridization between Wilsons Camellia (c. irrawadiensis) and C. sinensis var. assamica. Experimental F1 hybrids resembled the later in many morphological characters but were inferior in quality which was not accepted commercially.
A successful hybridization between C. sinensis and C. japonica has been reported by Bezbaruah and Gogoi (1972). F1 hybrids showed close morphological similarity with parents but yield and quality were low. Generally, hybridization with species of Camellia from which acceptable quality of tea can be manufactured have been encouraging. But there are exceptions. For example, a high yielding clone TV 24 in Assam produced from the crossing between F1 hybrids from C. irrawadiensis x C. assamica Shen and TV 2, an Assam- China hybrid (Bezbaruah, 1987).
There may be scope for introducing non-conventional approaches to tea improvement. But the potential of non- conventional methods is limited by lack of knowledge of some aspects of tea genetics.Mutation Attempts at inducing mutation in tea seeds, cuttings, pollen grains with physical (X-rays and G- rays) and chemical mutagens (Ethyl Methane Sulphonate) have not yielded desired success to produce superior mutants. Mutants produced by these treatments have stunted growth, reduced vigour, and a lesser number of foliage and branches (Singh, 1982).
The use of irradiated pollen also caused fruit drop (Satyanarayana and Sharma, 1993). The difference of responses to G-rays was also observed in tea clones. Cambod origin are comparatively less tolerant to G-rays than China and Assam origin (Singh, 1980). The apparent genetic variation in the response of mutagens suggests that the technique has potentiality in tea breeding.
Most of the cultivated tea all over the world is diploid (2n = 30). Natural triploids (2n = 45) resulting from open pollination are reported from Japan (Simura and Inabe, 1953), north-east India (Anonymous, 1970), south India (Venkataramani and Sharma, 1975), Georgia (Kapanadze and Eliseev, 1975); and tetraploids (2n = 60), pentaploids (2n = 75), Aneuploids (2n +- 1 to 29) from north-east India (Bezbaruah, 1968). These are commonly found in tea populations but in low frequency (Singh, 1982). Triploids and tetraploids had bigger leaf and havier shoots but poor in cup quality than diploids (Sarmah ahd Bezbaruah, 1984), and pentaploids and aneuploids were week bushes and poor rooters (Bezbaruah, 1968).
Density and morphology of sclereids and stomata proved to be useful indices in determining ploidy level, tetraploids shows highest density and size of sclereids and having the largest stomata. In south India, a natural triploid (clone UPASI- 3) is found to be vigorous, high yielder with overall quality and the clone UPASI- 20 which is another triploid reported to be moderate yielder having drought tolerant capacity (Satyanarayana and Sharma, 1993). Polyploid plants (16 triploids and 2 tetraploids) have also been identified among the east African tea clones (Wachira, 1994).
However, polyploids are in general vigorous and hardier than diploids but they do not always contribute high yield (Banerjee, 1992). Polyploids can be produced by treating with colchicine (Visser, 1969) and crosses between triploid and diploid clones. Polyploids with favourable characters could be cloned and used in the improvement of the genetic stock. In Sri Lanka, triploid clone HS-10A did well in very high altitudes compared to the diploid clones. Sundaram and TV-29 are triploid clones gaining popularity in India, Sri Lanka and Kenya.
The potential of tissue culture in various aspects of plant improvement have already been recognised and attracted the attention of scientists. Rapid multiplication of propagation materials through tissue culture, particularly in the initial selection, assumes importance in plant improvement programmes. Tissue culture offer opportunity to utilize genetic, physiological and biochemical procedures in developing ideotypes (Banerjee, 1986). It also holds promise in the development of pure lines through haploid technology, production of triploids through endosperm culture, culture of embryos of incompatible crosses and isolation of somaclonal variants (Singh, 1978).
Banerjee (1992) suggested further possibilities of genetic manipulation for production of pure line of tea for inbreeding, breeding and propagation of interspecific hybrids derived from the various combination of crosses by cotyledon and embryo culture, fusion of protoplast from different species to produce hybrid synkaryons which then could be regenerated into organised plants by using techniques of cell and tissue culture. Once the plants are regenerated through in vitro techniques, it is necessary to establish the genetic variability using Random Amplified Polymorphic DNAs in somaclonal plants. Since they are directly linked with DNA, these markers are more stable and are not influenced by the environmental conditions. (RajKumar et al., 1997).
Possibility of transferring low caffeine genes from other camellia sp. should be explored both through conventional hybridization and transgenic approaches to the modification of the biosynthetic pathway for caffeine synthesis, using antisense RNA technology to selectively switch off individual genes and block biochemical pathways. The other approach is through accelerated particle gene delivery (using gene gun) which has brought many recalcitrant crops within range for genetic manipulation (Lyer, 1995). Somatic cell hybridization using protoplast fusion has been tried to transfer the Darjeeling tea flavour of China clones to the Assam cultivars possessing strong and brisk liquors (Banerjee, 1986).
For early identification of blister bight resistance or susceptibility, SCAR (Sequence Characterized Amplified Regions) marker has been developed. This DNA marker will enable us in selecting blister blight resistant/ susceptible plants. Development of drought specific microsatellite markers (SSR) are in progress.
The collection of tea germplasm in India in fact began with the discovery of Assam tea in 1823. The collections were also made from Burma, Cambodia, China, Japan and Vietnam. At present over 1200 genetic stocks of tea including its wild species and weedy relatives are maintained at Tocklai Experimental Station (Bezbaruah and Dutta, 1977). This collection has undoubtedly helped India as well as other tea growing countries in evolving superior plant materials (Singh, 1979). The same have also been extensively used in hybridization (crossing) programmes for the development of clonal varieties (Bezbaruah, 1974) and its exploitation thus helped in broadening the genetic base of certain clones.
In south India, a majority of the selections available in the germplasm at present have been selected from some of the commercial tea estates of that region (Satyanallayana and Sharma, 1986). The preservation of tea germplasm has great importance primarily because of the seed-grown sections of tea are being massively uprooted and the seed sources of which no longer exist and lost forever. There is a vast genetic variability of plantations of Darjeeling hills as still over 95% of the tea area is seed originated and old.
The wide variability in the commercial tea populations offers scope for the selection of elite mother bushes with desirable attributes in practical plant improvement programme (Richards, 1966; Bezbaruah, 1975). Therefore, it needs to be preserved and utilized judiciously. Tea areas although at a slow rate are replanted with limited vegetative clones and jungles are also being cleared for extension of agricultural activities resulted narrowing the genetic base of Indian tea. Conservation of tea germplasm is done mainly by, establishing field gene banks comprising a wide array of indigenous and exotic cultivars will offer the necessary genetic diversity for the improvement of tea plant at a future date.